The proliverative response of antigen specific (for TNP) B lymphocytes to TI-2 antigen (eg TNP-Ficoll) shows an absolute requirement for BSF-1, and is enhanced by IL-1. Such responses to T dependent antigens (e.g. TNP-OVA) requires direct interation with carrier specific T cells as well as BSF-1. Such responses are suppressed by IFN. 2. A human BCGF. induced proliferation in resting murine small B lymphocytes, as well asaugmenting the response of such lymphocytes activated with dextran sulfate. In addition, the human BCGF augmented the response of murine cells to anti-Ig and BSF-1, although it was inactive in the absence of BSF-1. Human BCGF, not murine BCGF-II had this activity. Murine BCGF-II but not human BCGF stimulated BCL-1 lymphoma cells to division, suggesting the existence of two BCGF-II activities.